Microporous membranes are known, can be made by various processes, and the process by which the membrane is made has a material impact upon the membrane's physical attributes. See, Kesting, R., Synthetic Polymeric Membranes, A structural perspective, Second Edition, John Wiley & Sons, New York, N.Y., (1985). Three commercially viable processes for making microporous membranes include: the dry-stretch process (also known as the CELGARD process), the wet process, and the particle stretch process.
The dry-stretch process refers to a process where pore formation results from stretching the nonporous precursor. See, Kesting, Ibid. pages 290-297, incorporated herein by reference. The dry-stretch process is different from the wet process and particle stretch process. Generally, in the wet process, also know as the phase inversion process, or the extraction process or the TIPS process (to name a few), the polymeric raw material is mixed with a processing oil (sometimes referred to as a plasticizer), this mixture is extruded, and pores are then formed when the processing oil is removed (these films may be stretched before or after the removal of the oil). See, Kesting, Ibid. pages 237-286, incorporated herein by reference. Generally, in the particle stretch process, the polymeric raw material is mixed with particulate, this mixture is extruded, and pores are formed during stretching when the interface between the polymer and the particulate fractures due to the stretching forces. See, U.S. Pat. Nos. 6,057,061 and 6,080,507, incorporated herein by reference.
Moreover, the membranes arising from these processes are physically different and the process by which each is made distinguishes one membrane from the other. Dry-stretch membranes have slit shaped pores due to the inability to stretch the precursor in the transverse machine direction. Wet process membranes have rounder pores due to the ability to stretch the precursor in the transverse machine direction. Particle stretched membranes, on the other hand, are filled with particulate needed for pore formation. Accordingly, each membrane may be distinguished from the other by its method of manufacture.
While membranes made by the dry-stretch process have met with excellent commercial success, there is a need to improve their physical attributes, so that they may be used in wider spectrum of applications. Some areas of improvement include pore shapes other than slits and increase transverse direction tensile strength.
U.S. Pat. No. 6,602,593 is directed to a microporous membrane, made by a dry-stretch process, where the resulting membrane has a ratio of transverse direction tensile strength to machine direction tensile strength of 0.12 to 1.2. Herein, the TD/MD tensile ratio is obtained by a blow-up ratio of at least 1.5 as the precursor is extruded.